Prop fan engine propeller pitch control requires that in the event of loss of the blade pitch prime mover, blade pitch must be maintained in the realm of cruise setting, or advanced to "feather", i.e., where the blade is streamline to the air stream. In the absence of control from the blade pitch prime mover, windage tends to drive the blades to fine pitch, resulting in an immediate severe rotational engine overspeed, and very likely catastrophic failure of major rotating components. It is known to provide a no-back mechanism to prevent this back driving of the blade pitch in the event of loss of use of the blade pitch prime mover.
During normal operation of an adjustable pitch propeller, the blades will be subject to small pitch changes, "dither", on a regular basis. This is caused by a change in pitch induced by the air stream interacting with the propeller followed by a correction in the opposite direction induced by the blade pitch control system which tries to maintain a constant rotational speed of the blade assembly. In known no- o back mechanisms a brake is employed for resisting the back driving motion and the brake is typically activated and deactivated to perform this control reaction, that is, the brake is released in the decreasing pitch portion of the dither cycle. It has been found that this repetitive activation and deactivation of the brake with dither can result in premature wearing of the no-back mechanism brake.
Various types of pitch control mechanisms and no-back mechanisms are shown in U.S. Pat. Nos. 2,504,737; 2,947,364; 4,047,842 and 4,480,733.